Hydrogen ion selective sensor and electrode therefor

ABSTRACT

A hydrogen ion selective sensor has a sensing electrode with a palladium oxide coated palladium surface element and a reference electrode. A specific hydrogen ion selective sensor has a sensing electrode with a palladium oxide coated palladium surface element, a reference electrode spaced from and electrically insulated from the sensing electrode, an electrically insulating cover surrounding the palladium portion of the sensing electrode and the reference electrode, the cover defining an electrolyte chamber for containing a reference electrolyte therein, and a wick extending outwardly from the chamber adjacent the palladium oxide portion of the sensing electrode. An improved hydrogen ion selective sensing electrode is described which has a palladium oxide coated palladium surface element.

Jan. 9, 1973 [54] HYDROGEN ION SELECTIVE SENSOR Primary Examiner-T. TungAND ELECTRODE THEREFOR [75] Inventors: Willard T. Grubb; Lawrence H.

Attorney-Richard R. Brainard, Paul A. Frank, Charles Watts, Paul R.Webb, ll, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. FormanKing, both of Schenectady, NY.

General Electric Company ABSTRACT A hydrogen ion selective sensor has asensing elec- [73] Assignee:

trode with a palladium oxide coated palladium surface element and areference electrode. A specific hydrogen ion selective sensor has asensing electrode with a palladium oxide coated palladium surfaceelement, a reference electrode spaced from and electrically insulatedfrom the sensing electrode, an electrically insulating cover surroundingthe palladium portion of the sensing electrode and the referenceelectrode, the cover defining an electrolyte chamber for containing areference electrolyte therein, and a wick extending outwardly from thechamber adjacent the palladium oxide portion of the sensing electrode.An improved hydrogen ion selective sensing electrode is described whichhas a palladium oxide coated palladium surface element.

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This invention relates to hydrogen ion selective sensors and toelectrodes therefor and, more particularly, to hydrogen ion selectivesensors with hydrogen ion selective electrodes with a palladium oxidecoated palladium surface element.

Sensors are employed to determine the content of a specific substance ina fluid or atmosphere. For example, a sensor might be employed todetermine the content of oxygen, or carbon dioxide in a sample, or itscontent of hydrogen ions or other ions in solution.

lon selective sensors are known in the prior art for measuring thehydrogen ion activity or pH of a sample. Such a sensor employs areference electrode and a sensing electrode, such as a glass electrode,immersed in a solution, whereby the potential difference between the twoelectrodes is a function of the concentration of the hydrogen ion in thesolution. The reference electrode contains a salt solution. Electricalconnection between the salt solution and the sample solution is madegenerally by a liquid contact through an aperture referred to as aliquid junction.

Our present invention is directed to an improved miniaturized hydrogenion selective sensor and electrode therefor which is suitable forbiomedical application.

The primary objects of our invention are to provide a rugged, accurateand miniaturized ion selective sensor and hydrogen ion selectiveelectrode for use therein.

In accordance with one aspect of our invention, an ion selective sensorcomprises a hydrogen ion electrode which has a base member, an exteriorsurface of palladium on the base member, and palladium oxide adheringtightly to atleast a portion of the palladium surface; a referenceelectrode spaced from and electrically insulated from the hydrogen ionselective electrode, an electrically insulatingcover surrounding thepalladium surface portion of the hydrogen ion selective electrode andthe reference electrode, the cover defining an electrolyte chamber forcontaining a reference electrolyte in contact with the referenceelectrode, and a wick positioned partially in the chamber and partiallyoutside the cover adjacent the palladium oxide portion of the hydrogenion selective electrode.

These and various other objects, features and advantages of theinvention will be better understood from the following description takenin connection with the accompanying drawing in which:

FIG. 1 is a sectional view of a hydrogen ion selective electrode made inaccordance with our invention; and

F IG. 2 is a sectional view of a hydrogen ion selective sensor made inaccordance with our invention.

In FIG. 1 of the drawing, there is shown generally at a hydrogen ionselective electrode made in accordance with our invention. Electrode 10is shown in the form of an elongated base member 11 with an exteriorsurface of palladium 12 on base member 11, and palladium oxide 13adhering tightly to at least a portion of the palladium surface. Thepalladium surface 12 is also shown with a circular fluid-tightinsulation layer l4 of a suitable electrically insulating polymer bondedthereto. The insulated portion of palladium surface l2 extends into anadapter 15. The proximal end of elongated base member ll terminates in aconnector which is shown generally in 16. A suitable electrical lead(not shown) is connected to elongated base member 11 by being joined inconnector 16. The hydrogen ion selective electrode can be inserted inthe interior of a catheter for insertion within a body cavity, or usedfor clinical analysis.

In FIG. 2 of the drawing, there is shown generally at 20 an ionselective sensor made in accordance with our invention and employing ahydrogen ion selective electrode therein. Sensor 20 is shown with ahydrogen ion selective electrode 21 in the form of an elongated rod orwire. Electrode 21 is shown as a palladium base member which thereforhas the surface of palladium 22 thereon. Other base materials with apalladium surface deposited thereon can be employed. Adjacent one end ofelectrode 21 palladium oxide 23 adheres tightly to a portion of thepalladium surface. A layer of Alkanex polyester resin lacquer 24 coversthe palladium surface 22. This insulation 24 can also slightly overlapor be adjacent the palladium oxide 23. A reference electrode 25 in theform of a silver wire is applied to the outer surface of insulation 24and provides the reference electrode for the sensor. The lower end ofreference electrode 25 is provided as at 26 with a coating of silverchloride. The opposite end of electrode 25 extends outwardly from thedevice in connection to an electrical lead (not shown). In this mannerreference electrode 25 is spaced and electrically insulated from sensingelectrode 21.

An electrically insulating cover 27 of a suitable material such as heatshrinkable polyol efin tubing surrounds the palladium surface portion 22of sensing electrode 21, and reference electrode 25 with its silverchloride coating 26. Cover 27 defines an electrolyte chamber 28 whichchamber is in contact with reference electrode 25. The upper end ofcover 27 is shown as open while the lower end of cover 27 is fittedtightly, for example by heat shrinking, as at 29 around inner insulation24 to form a closure except at one point 30 which provides the liquidjunction. A wick 31 is positioned partly in the electrolyte chamber andpartly outside or flush with cover 27 adjacent the palladium oxideportion 23 of sensing electrode 21. An electrolyte 32 is containedwithin the chamber defined by cover 27.

We found that we could form an improved hydrogen ion selective electrodewhich electrode was also useful in a hydrogen ion selective sensor. Wefound that we could provide such an electrode from a base member whichhad a surface of palladium thereon. Such an electrode requires furtherthat palladium oxide adhere tightly to at least a portion of thepalladium surface. We found that such a base member may be made of anynumber of materials which can withstand the high temperature treatmentto provide the palladium oxide coating. We further found that any numberof metals could be employed for the base member to which a surface ofpalladium could be applied. In its preferred form we, of course, providethe base member in the form of palladium metal whereby it has a surfaceof palladium.

We found that various disk, rod, wire or irregular shaped hydrogen ionselective electrodes could be prepared in accordance with our invention.pH measurement can be carried out in various ways with such electrodes.Wire-shaped electrodes can be dipped into a solution to be measuredeither with an associated reference electrode or in the above sensorstructure. Rod shaped electrodes can be mounted through the wall of atube carrying a stream of liquid to monitor the pH. Disk-shapedelectrodes can form the bottom of a cup into which a sample of liquidfor pH measurement is placed.

As distinct from glass pH electrodes, the present electrodes possess theadvantage that they are rugged, easily miniaturizable, can be readilyfabricated in a wide variety of shapes and sizes, and possess a lowimpedance voltage signal to the measuring circuit. The volt meter usedfor measuring the voltage of our hydrogen ion selective electrode todetermine the pH of a sample can be as low as about megohms. Glasselectrodes require ordinarily electrometer type meters with inputimpedances in the range of 1 million megohms or greater.

We discovered unexpectedly that we could form an improved hydrogen ionselective electrode. As it was referenced above the copendingapplication Ser. No. 33,198 described and claimed an iridium-iridiumoxide electrode. While it would appear that any of the four Group VIIIof noble metals available in wire form could be employed as an electrodeby providing the respective oxide thereon, this does not occur. Bothrhodium and platinum can have an oxide layer applied thereon but suchoxide layer is visually different in that it is tan in color and quitedifferent in appearance from the nearly black coating formed ofpalladium oxide on a palladium surface. The rhodium wire which had beenoxidized did not sense pH at all. The platinum wire which had beenoxidized produced a voltage change in the right direction between pH 7and 8 but with an exceedingly long time constant. Thus neither rhodiumnor platinum can have its respective oxide applied thereto and functionusefully as a hydrogen ion selective electrode. in contrast to this wefound that our hydrogen ion selective electrode functioned veryeffectively for the measurement of pH.

We found unexpectedly that we could employ our above hydrogen ionselective electrode as the sensing electrode to provide an improvedhydrogen ion selective sensor. For example, such a sensor can be formedby employing an elongated hydrogen ion selective electrode with apalladium oxide coated palladium surface element, and a referenceelectrode spaced from and electrically insulated from the sensingelectrode. The preferred reference electrode is a silver wire with asilver chloride coating on one end. We found that the electricalinsulation on the palladium surface portion can be made from a varietyof materials such as Alkanex polyester resin lacquer, polyphenyleneoxide, Viton hexafluoropropylene-vinylidene fluoride rubber, siliconerubber, Lucite plastic, etc. An electrically insulating cover or tubesurrounds both the palladium surface portion of the sensing electrodeand the reference electrode. Similarly, this cover or tube may be madefrom a wide variety of electrically insulating materials. We found,however, that a polyolefin which is heat tory manner. For example, thetube can be slipped over the two electrodes and the end of the tubeadjacent the palladium oxide heat shrunk to bond the tube at its one endto the first layer of insulation. The cover provides an electrolytechamber which is in contact with the reference electrode. A wick ispositioned partially in the chamber and partially outside or flush withthe cover adjacent the palladium oxide portion of the hydrogen ionselective electrode to provide a liquid contact structure. The chamberis normally filled with a reference electrolyte such as potassiumchloride. The wick is preferred to be a length of multifilamentarythread such as glass thread.

We have found that we can prepare the hydrogen ion selective electrodeby employing the high temperature reaction of palladium with oxygen fromthe air in the presence of an alkali metal hydroxide. The parameters ofpreparing such an oxide electrode were investigated as to the choice oftime and temperature of the reaction, the choice of the alkali metalhydroxide and its concentration, precleaning and roughening of thepalladium base member and the use of electrical insulation on theportion of the base member. We found that treating the exteriorpalladium surface of the base member at a temperature of 800C appears tobe the optimal temperature. However, palladium oxide can be formed onsuch a surface as low as 350C and up to about 938C where the oxidedecomposes. Of the alkali metal hydroxides tested, sodium hydroxide gavethe best electrode with potassium hydroxide providing good electrodes.We found that lithium hydroxide did not appear to work. In theprocess-precleaning and roughening of the exterior palladium surface ofthe base member was found to be required for best results. Suchprecleaning and roughening was accomplished by dipping in aqua regia forabout 30 seconds or sand blasting or a combination of both. The heatingperiod at 800C was varied from about 25 seconds to minutes. Within thistime period the optimal time was l0-2O minutes. When electricalinsulation'was applied on the exterior palladium surface of the basemember after oxidation of the other portion of the surface of the basemember to a palladium oxide surface the results were more reproducible.

A preferred method of forming a hydrogen ion selective electrode made inaccordance with our invention was to employ an elongated base member ofpalladium which was dipped at one end into the aqua regia for 30 secondsto etch and preclean the surface. The cleaned end was rinsed and thendipped into a 50 weight percent solution of sodium hydroxide. The sameend of the elongated base member was then placed for 20 minutes in thehot zone of a furnace controlled at 800C. The palladium base member witha black palladium oxide coating portion adhering tightly to the one endof the palladium base member was removed from the furnace, rinsed inflowing distilled water and stored in distilled water or a neutralbuffer solution. Subsequently, the electrode was rinsed, dried and alayer of electrical insulation of Alkanex lacquer was applied from theupper end of the palladium oxide coating to near the opposite end of thebase member. After drying, the Alkanex lacquer was cured. A segment ofuninsulated base member was provided at the opposite end to provide fora connection of an electrical lead subsequently thereto. This methodresults in the production of an improved hydrogen ion selectiveelectrode made in accordance with our invention.

The hydrogen ion selective sensor made in accordance with our inventioncan be formed preferably by employing a hydrogen ion selective electrodemade as above described. in the above preparation the pa]- ladium basemember with palladium oxide on one end has applied to its palladiumsurface portion a layer of Alkanex polyester resin lacquer which hasbeen coated thereon from a solution containing a cresol solvent. Thesolvent is then evaporated and the Alkanex lacquer is cured. A chloridedsilver wire is then applied to the outside of the Alkanex lacquerinsulation from a point adjacent the palladium oxide coating andextending therefrom along the length of the insulation. A heatshrinkable tube of polyolefin was placed around the two electrodesseparated by the first insulation to thereby form a concentric chamber.The end of the tube near the palladium oxide coating was heat shrunk tobond with the first electrical insulation except for a small openingtherein containing a multifilament fiber.

The fiber or wick is positioned initially against the exterior surfaceof the first electrical insulation so that when the cover is heat shrunkon to the first insulation the wick will be positioned partially withinthe cover and partially outside or flush the cover adjacent thepalladium oxide portion of the sensing electrode. in this manner thecover defines a chamber which contains the reference electrode. Areference electrolyte such as 0.1 to 4 normal potassium chloridesolution was placed in the chamber by inserting the end of the sensor ina test tube filled with the solution. Over a 24 hour period a sufficientamount of the solution entered the chamber through the wick junction.Similarly, the filling can be accomplished by use of a syringe. Theopposite end of the sensor comprises an uninsulated portion of thesensing electrode and a portion of the reference electrode which areconnected to suitable leads so that a voltage can be measuredthereacross. This resulted in a hydrogen ion selective sensor made inaccordance with our invention.

Examples of hydrogen ion selective electrodes and hydrogen ion selectivesensors employing such electrodes made in accordance with our inventionare as follows:

EXAMPLE 1 A hydrogen ion selective electrode was prepared as describedabove. A portion of a palladium wire 30 mils in diameter was etchedbriefly in aqua regia, rinsed in distilled water, dipped in-50 percentaqueous sodium hydroxide solution and heated in an oven controlled at800C for minutes. During the heat-up, the sodium hydroxide became moltenand distributed itself in an even manner over the lower end of the wireat the same time the formation of a black coating of palladium oxide wasevident. This black coating was observed to creep up the wire as themolten sodium hydroxide wetted the wire and spread upward under theinfluence of surface tension forces. The electrode was cooled, rinsed inwater and stored in a pH 7.4 phosphate buffer.

After 2 daysstorage, this electrode was tested in a series of phosphatebufiers over the pH range of 6.0 to

8.0. The results of these tests are shown below in Table l in which islisted the voltage against a reference electrode, Ag-AgCl in saturatedKCl as a function of pH. Response time was 30 seconds or less for a 1unit change in pH, and there was no hysteresis within 1 millivolt as thepH was varied in the order 7, 8, 7, 6, 6.6, 7, 7.6, 8. The selectedvalues were stable within about 1 millivolt for 1 hour or more. i

The slope in millivolts per pH unit is very close to the theoreticalslope of 59 millivolts per pH unit.

TABLE I Millivolts Versus pH Reference Electrode mV 6.0 358 7.0 300 8.0240 EXAMPLE 2 Thirty-four palladium-palladium oxide electrodes were madeas described above. Each electrode was made as follows: A 30 mildiameter palladium wire 4 inches in length was dipped one-half inch onone end into aqua regia for 30 seconds to etch and preclean the surface.The cleaned end was rinsed; then dipped into a 50 weight percentsolution of sodium hydroxide. This end was placed for 20 minutes in thehot zone of a furnace controlled at 800C. The wire with its black oxidecoating of palladium oxide on one end was then removed, rinsed inflowing distilled water for 30 seconds and stored in a dilute pH 7.4phosphate buffer.

Most of the electrodes were subsequently rinsed, dried, and insulatedfrom the upper end of the palladium oxide coating to near the oppositeend of the wire. An uninsulated section onthat end was left forelectrical connection. Two insulating methods were employed. Apolyolefin shrink tube was used for one electrode while a solution ofpolymethyl methacrylate in ethylene dichloride was painted on with asoft brush and air dried to provide the other 33 electrodes.

The 34 electrodes were tested in the following manner. Each electrodewas removed from its storage solution, immersed in a pH 7 buffer and itsvoltage measured against a silver-silver chloride, saturated KCl, fiberjunction reference electrode. The voltage was recorded on a recordingmillivoltmeter. Readings were taken upon reaching steady state asdefined by the criterion of no more than 0.1 millivolt drift in 1minute. This corresponded to a drift of slightly less than 0.002 pHunits per minute. Such a drift rate is in keeping with the highprecision required for pH measurement in clinical laboratories and inpatient monitoring. After steady state was reached in the pH 7 buffer,the electrode and reference were rinsed with distilled, deionized waterand immersed in pH 8 buffer. The steady state reading was taken asbefore, and the time constant for the electrode was estimated. Thesteady state readings on the present electrodes were achieved inunstirred solutions in times of 1 minute or less, often only a fewseconds. Following the pH 8 reading, the electrode was returned to pH 7buffer and remeasured. Correspondence within 1 millivolt to the initialreading was achieved in a preponderance of the electrodes measured.

Out of the 34 electrodes tested in the above manner, two were rejecteddue to excessive drifting in the readings at the initial pH. Theremaining 32 electrodes represented the basis for a brief statisticalanalysis.

' Their reproducibility in voltage at pH 7 and in the millivolts changeper pH unit between pH 7 and 8 was determined.

The above results are summarized below. In Table ll the voltage is shownof a single electrode from pH 2 to about pH 9. The theoretical potentialpH line for the reaction of palladium with water to form palladium (ll)oxide monohydrate is a slope of 59.2 mV per pH unit. The average slopebetween pH 7 and 8 for 32 electrodes was 56.3 mV/pH unit with a standarddeviation of l.l millivolts. Thus, an electrode with a single pointcalibration at pH 7.5 would read at pH 7.0 and pH 8.0 to an accuracy of$0.02 pH units at twice the standard deviation in slope for this groupof electrodes. The mean voltage of the 32 electrodes at pH 7 was 273.5with a standard deviation of 14.5 mV. For the reaction Pd 2H O PdO'l-l O2H 2e, a calculated value of 283 mV would be observed at pH 7.

A hydrogen ion selective sensor was made in accordance with ourinvention as above described. The hydrogen ion selective electrode was a30 mil diameter wire of palladium coated on one end with palladiumoxide. The palladium portion of the electrode was insulated with Alkanexpolyester resin lacquer from a cresol solvent after which the solventwas evaporated and the Alkanex polyester resin lacquer was cured. Theinsulation overlapped slightly the palladium oxide portion of theelectrode. A 2 mil diameter silver wire coated on one end with silverchloride was applied to the exterior surface of the Alkanex polyesterresin lacquer and extended from adjacent the palladium oxide portion'ofthe hydrogen ion selective electrode along the length of the insulation.A piece of heatshrinkable polyolefin tube with an internal diameter of60 mils was positioned around the sensing electrode first insulation andreference electrode. A short length of a multifilamentary glass threadwas positioned adjacent the end of the first insulation so that aportion of the thread or wick was within the polyolefin tubing and aportion extended outwardly adjacent the palladium oxide portion of thesensing electrode. The end of the tubing adjacent the palladium oxideportion of the hydrogen ion selective electrode was shrunk to sealagainst the insulated palladium wire by heating that portion of thepartially assembled structure to about 120C for 1 minute. in this mannerthe wick extending from the polyolefin tubing provided a liquidjunction. An annular reference electrolyte chamber was defined betweenthe interior surface of the tubing and the insulated portion of thepalladium wire. This chamber was filled with about 50 microliters of 0.1normal potassium chloride solution by inserting the wick and adjacentstructure of the sensor into a test tube with the potassium chloridesolution for a period of 24 hours.

The assembled device was a hydrogen ion selective sensor made inaccordance with our invention. The overall length of the sensor was 4inches and the maximum outer diameter was 0.070 inches. The electrodewas tested as series of buffers to determine its characteristics. Thesedata are set forth below in Table 11]. The voltage was a good linearfunction of pH with a slope of 58.1 millivolts per pH unit at 24C. Theresponse to pH changes was such that steady readings were obtained after2 minutes or less.

TABLE Ill Millivolts Versus pH For pH pHReference Electrodes -mV 2.0 4353.0 385 4.0 325 5.0 270 6.0 210 7.0 150 8.0 9.0 45

EXAMPLE 4 A hydrogen ion selective electrode was prepared in which itwas desired to confine the palladium oxide on the palladium wire to anexactly determined area of the wire. The palladium wire which was 30mils in diameter was gold plated with 0.0025 inch of gold. The end ofthe gold plated coated wire was then sand blasted for a length ofone-sixteenth of an inch from one end of the wire to remove the goldplating therefrom. The one-sixteenth of an inch of the wire was dippedin 50 weight percent aqueous sodium hydroxide. The structure was driedfor 5 minutes in dry nitrogen which was flowed at a rate of 10 cubicfeet per hour in a half liter jar. The structure was then oxidized for15 minutes in an oven controlled at 800C with an oxygen flow of onecubic foot per hour. The one-sixteenth of an inch of the wire withoutthe gold plate formed a black coating of palladium oxide thereon. Theelectrode was then rinsed for 15 minutes in water and then stored inwater for 20 hours. Subsequently, the electrode was dried for 2 hours inan air atmosphere. Three coatings of Alkanex polyester resin lacquerwere applied over the gold plated portion of the electrode. After eachcoating of Alkanex polyester resin lacquer, the lacquer was dried andcured. The resulting structure was a hydrogen ion selective electrodemade in accordance with our invention.

This electrode showed an extremely fast response time. Between pH 7 andpH 8 the response time was about 1 second and the change of voltage was58.5 mV.

EXAMPLE 5 A hydrogen ion selective electrode was prepared as inExample 1. The heating time at 800C was 15 minutes. After the palladiumoxide coating had formed on a portion of the exterior of the palladiumwire the electrode was rinsed for .15 minutes in water and then soakedovernight in water. The following morning the electrode was dried andsubmitted for metallographic examination where the electrode wascross-sectioned and metallographic pictures were taken. A coating ofpalladium oxide on the palladium wire was remarkably uniform, beingbetween 0.3 and 0.4 thousandths of an inch and accurately following thesurface contours of the palladium wire substrate.

EXAMPLE 6 A hydrogen ion selective electrode was prepared by employing a30 mil diameter Nichrome wire which was then plated with a 0.0095 inchthickness of palladium. One-half inch of this wire at one end was dippedin 50 percent aqueous sodium hydroxide solution. The structure was driedfor minutes in dry nitrogen which was flowed at a rate of 10 cubic feetper hour in a half liter jar. The structure was then oxidized forminutes at 800C with an oxygen flow of 1 cubic foot per minute whereby ablack coating of palladium oxide was formed on the one-half inch of thewire. After rinsing for 15 minutes in water the electrode was soakedovernight in water. The exterior palladium surface was provided withelectrical insulation by painting on 50 weight percent room temperaturevulcanizing silicone from a solvent. This electrode was then tested in aseries of buffers and produced a voltage change of 57.0 mV per pH unitin the range of pH 7 to pH 8.

EXAMPLE 7 A hydrogen ion selective electrode was prepared by welding aflat disc of palladium metal 0.25 inch in diameter by 0.030 inch inthickness to a platinum wire having a diameter of 0.030 inch. Theexterior face of the palladium disc was sand blasted after which it wasdipped into a 50 percent aqueous sodium hydroxide solution. Thestructure was then dried for 15 minutes in dry air. The structure wasthen oxidized for 15 minutes in an oven at 800C. A black coating ofpalladium oxide formed on the exterior surface of the plate. Afterrinsing for 15 minutes in running water, the electrode was soaked inwater overnight. No insulation was applied. The electrode was tested forits response to pH which was 56.3 mV per pH unit in the range of pH 7 topH 8.

While other modifications of the invention and variations thereof whichmay be embraced within the scopeof the invention have not beendescribed, the invention is intended to include such as may be embracedwithin the following claims.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

l. A hydrogen ion-selective sensor comprising a hydrogen ion-selectiveelectrode in the form of an elongated wire, said electrode comprising abase member, an exterior surface of palladium on the base member, andpalladium oxide adhering tightly to at least a portion of the palladiumsurface; a reference electrode spaced from and electrically insulatedfrom the hydrogen ion-selective electrode, an electrically insulatingcover surrounding the palladium surface portion of the hydrogenion-selective electrode and the reference electrode, the cover definesan electrolyte chamber in contact with the reference electrode, and awick sitioned partially in the chamber and partially outsi e the coverad acent the palladium oxide portion of the hydrogen ion-selectiveelectrode.

2. A hydrogen ion selective sensor as in claim 1, in which theelectrically insulating cover is polyolefin heat-shrinkable tubing.

3. A hydrogen ion selective sensor as in claim 1, in which a referenceelectrolyte is contained in the chamber.

2. A hydrogen ion selective sensor as in claim 1, in which theelectrically insulating cover is polyolefin heat-shrinkable tubing.
 3. Ahydrogen ion selective sensor as in claim 1, in which a referenceelectrolyte is contained in the chamber.